
Gross Pitch Error  [1]} - the percentage of frames for which the absolute pitch error is higher than a certain threshold. For speech, this threshold is usually 20% where pt,\(p^{\prime }\textsubscript {t}\)  are the pitch signals from the reference and predicted audio, vt,\(v^{\prime }\textsubscript {t}\)  are the voicing decisions from the reference and predicted audio, and \(\mathbb {1}\)  is the indicator function.
\(GPE = \dfrac{\sum _{t}\mathbb {1}[|p\textsubscript {t} - p^{\prime }\textsubscript {t}| - 0.2p\textsubscript {t}]\mathbb {1}[v\textsubscript {t}]\mathbb {1}[v^{\prime }\textsubscript {t}]}{\sum _{t}\mathbb {1}[v\textsubscript {t}][v^{\prime }\textsubscript {t}]}\) 

Voicing Decision Error  [1]} - The percentage of frames for which an incorrect voiced/unvoiced decision is made where vt,\(v^{\prime }\textsubscript {t}\)  are the voicing decisions from the reference and predicted audio, and \(\mathbb {1}\)  is the indicator function and T is the total number of frames.
\(VDE = \dfrac{\sum _{t=0}^{t=T-1}\mathbb {1}[v\textsubscript {t}\ne v^{\prime }\textsubscript {t}]}{T}\) 

F0 Frame Error  [3]} - percentage of frames where either a GPE or VDE is observed.
\(FFE = \dfrac{\sum _{t}\mathbb {1}[|p\textsubscript {t} - p^{\prime }\textsubscript {t}| - 0.2p\textsubscript {t}]\mathbb {1}[v\textsubscript {t}]\mathbb {1}[v^{\prime }\textsubscript {t}] + \mathbb {1}[v\textsubscript {t}\ne v^{\prime }\textsubscript {t}]}{T}\) 

Mel Cepstral Distortion  [4]} - It is the measure of how different two sequences of mel cepstra are. It is used in assessing the quality of parametric speech synthesis systems Where ct, \(c^{\prime }\textsubscript {t}\)  are the k-th mel frequency cepstral coefficient (MFCC) of the t-th frame from the reference and predicted audio. We sum the squared differences over the first K MFCCs
\(MCD\textsubscript {K} = \dfrac{1}{T} \sum _{t=0}^{T-1}\sqrt{\sum _{k=1}^{K}(c^{\prime }\textsubscript {t,k} - c\textsubscript {t,k})}\) 

